This should support the following uses:

• better support for relocated addresses (eg when using function sections, which have every function symbol at address 0)
• reuse of the same address type for conversion when both reading and writing
• optional support for segmented addresses (Support non-zero segment_selector_size #357)
• using the native address size
• better type checking of our address handling

I wanted this when doing the relocated address support, but didn't do it because of how ugly the Reader::Offset support had been. However, I don't think that's so bad now after the default offset was changed in #392.

Any thoughts on this @fitzgen ?